A street lighting device of the most commonly used type (such as a lamp post) comprises a structure 10 which supports at a certain height a light source 12 intended to project light radiation downward toward the street plane S.
As shown schematically in FIG. 1 of the appended drawings, this form of lighting is such that, in conditions of reduced visibility, for example in the presence of fog or other atmospheric precipitation such as rain, snow or smoke, the lighting of the street plane S is far from optimal. It may even be the case that visibility at the level of the street plane S is worsened by the effect of the lighting. Even on lighted stretches of road, drivers may therefore always prefer to use any fog lamps which are available on their vehicles.
The inventors have observed that any worsening of visibility as a result of the switching on of street lighting is due to the fact that, in these conditions, the degree of visibility of the street plane S by an observer O (such as a driver) can be modeled as the superimposition of two components, namely:                a “signal” component, corresponding to the light emitted from the observed scene, which diffuses (back) toward the observer O the light radiation originating from the source 12 and from any vehicle headlights that may be present;        a “noise” component, corresponding to the light from the source 12 diffused by diffusion sources DS such as fog droplets, raindrops, snowflakes or particles of smoke.        